What version of pandas are you using? For me your code works fine (i`m on git master).
Another approach could be:
In [117]: import pandas
In [118]: import random
In [119]: df = pandas.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))
In [120]: rows = random.sample(df.index, 10)
In [121]: df_10 = df.ix[rows]
In [122]: df_90 = df.drop(rows)
Newer version (from 0.16.1 on) supports this directly:
http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.sample.html